It could not be determined if the left upper aft cowling latches were secured properly before the aircraft departed. It is considered probable, however, that the cowling would have opened sooner if the latches had not been engaged before take-off, as there is normally a pressure differential across the cowling that tends to force it open. The rear latch was misaligned following the accident and wear patterns indicated that the condition had existed for some time. This discrepancy would have made it more difficult to operate the rear latch, and would have increased the likelihood of the rear latch being improperly secured when the cowling was closed. Testing demonstrated that differential air pressure could disengage the trigger on the forward latch because of the weak trigger spring. If the front latch disengaged in flight, as had occurred on at least one previous occasion, the front of the cowling may have lifted as the airspeed increased during the descent. The rear latch could have subsequently disengaged because of the effect of ram air flow in the accessory compartment or because it was not secured properly to begin with. The detached cowling lodged on the leading edge of the left wing immediately forward of the outboard end of the left elevator. The buffeting generated by the displaced cowling was sufficient to excite a destructive vibration in the elevator. There may have been pre-existing discrepancies in the vicinity of the failure; however, no such condition was identified on the components available for examination. The following Engineering Branch reports were completed: LP 138/95 - Performance Analysis LP 173/95 - Engine Cowl Latch AssemblyAnalysis It could not be determined if the left upper aft cowling latches were secured properly before the aircraft departed. It is considered probable, however, that the cowling would have opened sooner if the latches had not been engaged before take-off, as there is normally a pressure differential across the cowling that tends to force it open. The rear latch was misaligned following the accident and wear patterns indicated that the condition had existed for some time. This discrepancy would have made it more difficult to operate the rear latch, and would have increased the likelihood of the rear latch being improperly secured when the cowling was closed. Testing demonstrated that differential air pressure could disengage the trigger on the forward latch because of the weak trigger spring. If the front latch disengaged in flight, as had occurred on at least one previous occasion, the front of the cowling may have lifted as the airspeed increased during the descent. The rear latch could have subsequently disengaged because of the effect of ram air flow in the accessory compartment or because it was not secured properly to begin with. The detached cowling lodged on the leading edge of the left wing immediately forward of the outboard end of the left elevator. The buffeting generated by the displaced cowling was sufficient to excite a destructive vibration in the elevator. There may have been pre-existing discrepancies in the vicinity of the failure; however, no such condition was identified on the components available for examination. The following Engineering Branch reports were completed: LP 138/95 - Performance Analysis LP 173/95 - Engine Cowl Latch Assembly No physical evidence was found to indicate whether the latches were engaged before flight. The aircraft was fitted with early production Part No. H296K854 cowling latches that have weaker trigger springs than the current version Part No. H296K1135 latches. The design of the latches is such that a pressure differential across the latches results in a force on the latches in the direction in which they open. The forward latch had reportedly unlatched in-flight previously. Testing determined that the forward latch could be triggered open by a differential pressure equal to that present across the cowling in flight. Wear patterns indicated the rear latch may have been misaligned for some time, which would have made it more difficult to operate. The left elevator tip failed as the result of a severe up/down bending vibration that was induced by buffeting from the displaced cowling.Findings No physical evidence was found to indicate whether the latches were engaged before flight. The aircraft was fitted with early production Part No. H296K854 cowling latches that have weaker trigger springs than the current version Part No. H296K1135 latches. The design of the latches is such that a pressure differential across the latches results in a force on the latches in the direction in which they open. The forward latch had reportedly unlatched in-flight previously. Testing determined that the forward latch could be triggered open by a differential pressure equal to that present across the cowling in flight. Wear patterns indicated the rear latch may have been misaligned for some time, which would have made it more difficult to operate. The left elevator tip failed as the result of a severe up/down bending vibration that was induced by buffeting from the displaced cowling. It is probable that the left cowling opened in flight because of the combination of weak latch trigger springs and pre-existing damage on the rear latch. The left elevator failed because of buffeting induced by the displaced cowling.Causes and Contributing Factors It is probable that the left cowling opened in flight because of the combination of weak latch trigger springs and pre-existing damage on the rear latch. The left elevator failed because of buffeting induced by the displaced cowling. As a result of this occurrence, Contact Air has made the following change to the Company Standard Operating Procedures: When possible all night flight walk-arounds are to be completed inside the hangar, with all necessary hangar lighting on. This assists the crew to prepare the aircraft for flight and eliminates the need to use a flashlight for the walk-around.Safety Action Taken As a result of this occurrence, Contact Air has made the following change to the Company Standard Operating Procedures: When possible all night flight walk-arounds are to be completed inside the hangar, with all necessary hangar lighting on. This assists the crew to prepare the aircraft for flight and eliminates the need to use a flashlight for the walk-around.